1. Field of the Invention
This invention is related to regulators and, more particularly, to a regulator circuit that can efficiently power a plurality of loads from a power source.
2. Description of Related Art
FIG. 1 depicts a conventional Alternating Current (or AC) driven Direct Current (or DC) load 102. As illustrated, a bridge rectifier 108 rectifies an incoming AC line voltage in a well-known manner with the resulting full-wave rectified AC voltage applied to the DC load 102, which is a series combination of individual DC loads 106 with substantially similar electrical characteristics in a DC load string 104 and current limiting resistor R. The current limiting resistor R is needed to control the magnitude of the current in the DC load string 104, and to keep the current within safe operating limits.
In general, the DC load 102 of the circuit topography of FIG. 1 is specifically designed to work with a particular magnitude of incoming AC line voltage Vac(t). That is, the DC load 102 is generally sensitive to incoming AC line voltage transients or potential volatilities or variations thereof. Accordingly, the DC load 102 is selected to function within a specific operating voltage range, requiring careful matching of an appropriate DC load string 104 for a given operating region (or magnitude of incoming AC line voltage Vac(t)). Non-limiting examples of magnitudes of incoming AC line voltage Vac(t) may include 100V, 110V, 120V, 220V, 230V, 240V, or any others dependent on application. Misapplication of the magnitude of incoming AC line voltage Vac(t) and the DC load string 104 requirements can cause many problems, including shortened life of the individual DC loads 106, or insufficient operation of the DC load string 104.
Regrettably, the use of specifically designed DC loads 102 for a given incoming AC line voltage range results in manufacturing difficulties and restricted use of the DC load 102. For example, the DC loads 106 in the DC load string 104 may need to be specifically measured and binned during the manufacturing process, and application of a given DC load string 104 and resistor R combination must be restricted for use with a specified limited AC line voltage range. Therefore, in applications employing the circuit of FIG. 1, DC loads 102 are manufactured with specific electrical characteristics so as to enable operation at specified magnitude of incoming AC line voltage Vac(t). Accordingly, the required manufacture of DC loads 102 with specific or particular electrical characteristics to operate within a specifically commensurate magnitude of incoming AC line voltage Vac(t) becomes costly, and can lead to premature failure of the DC load 102 if misapplied by the consumer.
It should be noted that the choice of the resistor value of the current limiting resistor R of the circuit of FIG. 1 depends on many factors, including the selected magnitude of the incoming AC line voltage Vac(t), and characteristics of the DC load string 104, such as the number of DC loads 106 used in the DC Load string 104, the individual DC load 106 voltages (or the total DC load string 104 voltage), efficiency desired, and others. In order to maintain high efficiency (i.e., maximum operational output and minimum power dissipation), the resistor value of the current limiting resistor R, number of DC loads 106, and DC load voltage of each DC load 106 are chosen to yield maximum operational output with minimum power dissipation in the current limiting resistor R, while ensuring that safe current levels are maintained in order to protect the DC loads 106 from damage. This typically leads to selecting a specific number of DC loads 106 with operational voltage requirements that are commensurate with the peak input voltage (i.e., slightly lower than the peak voltage to further account for the cumulative voltages across the bridge rectifier 108 and the current limiting resistor R).
The current limiting resistor R is particularly useful when used with non-linear DC loads 106 wherein the relationship between the DC loads currents and DC loads voltages is non-linear (e.g., exponential). A non-limiting example of a DC load with non-linear electrical characteristics (e.g., non-linear current and voltage relationship) is a Light Emitting Diode (LED). LEDs are increasingly used as a source of light, in a wide variety of applications, including domestic and industrial lighting, traffic signaling, and decorative lighting, to name a few. The reasons for this proliferation are many: LEDs are efficient, rugged, and can operate over wide variations in temperature. Owing to their higher efficiency, LED-based light bulbs currently available can produce the same light output, with only a fraction of the input power of equivalent incandescent bulbs. Furthermore, LEDs have great reliability, making for light sources with extended lifespan. Additionally, with the proliferation of these light sources and the benefits of high volume production, they are becoming increasingly cost effective.
FIG. 2A depicts a conventional AC driven power circuit, with DC load 102 including a DC load string 104 that is comprised of non-linear DC loads 106 in the form of LEDs. FIG. 2B is an exemplary graphical illustration of typical electrical characteristics, also known as I-V characteristics, of the DC load 102 and DC load string 104 shown in FIG. 2A. As illustrated in FIG. 2B, the application of voltage across an LED string generates an exponential increase in the current through it, which, if increased above a designed limit, Ion(max), may easily damage the LED string. Most devices, linear or non-linear, are designed to operate within a very specified range of current. The current limiting resistor R is used to “linearize” the relationship between voltage and current in the load, so that when the voltage increases, the current in the load will be limited, and may be increased incrementally rather than exponentially. Coupling resistor R to the DC load 104 string extends the useful operating voltage range of DC load 104, from Von(min) to Von(max), as illustrated in FIG. 2B. This benefit does not come without cost, as resistor R is dissipative, and therefore lowers the efficiency of the system, and may lead to thermal problems if the power dissipated is too large.
In addition to the abovementioned design, manufacturing, and deployment issues (e.g., matching electrical characteristics of the DC load 102 requirements with the incoming AC line voltage Vac(t) and so on), the circuits of FIGS. 1 and 2A have a relatively low power factor. Power factor may be thought of as a measure of the effectiveness with which power is delivered to a load from an AC power source. As best illustrated in FIGS. 2C and 2D, because the DC load string 104 in FIGS. 1 and 2A requires a minimum turn ON voltage Von(min) before the individual DC loads 106 conduct significant current, the AC current (as a result of the applied AC voltage Vac(t)) flows only during brief intervals around the peaks of the AC line cycle (where sufficient voltage Von(min) illustrated in FIG. 2C is available to generate sufficient current Idc(t) to turn ON DC load string 104). Accordingly, as the incoming voltage Vac(t) increases (or ramps up) from zero to Von(min), essentially no current or power is actually delivered to the loads during this period. In other words, there is a significant time period in the AC line cycle during which the DC load string 104 is OFF. Stated otherwise, the DC load string 104 is only ON in intervals during which the magnitude of the incoming AC voltage Vac(t) is equal to or greater than Von(min), and the DC load string 104 is OFF in others, resulting in pulsed operation of the DC load string 104. This results in poor power factor, as indicated by the pulse current waveform Idc(t) in FIG. 2D, where current flows only in the intervals between Ton to Toff, with high peak current Ipk 1.
Also shown in FIG. 2D is an ideal current waveform with unity power factor, having substantially reduced peak current Ipk 2. In order to achieve the same average current needed to generate the desired DC load output, the peak current Ipk 1 of the pulsed current waveform must be considerably higher than it would be under higher power factor conditions like the ideal current waveform with peak current Ipk 2. The result of this pulsed current waveform is high peak current and poor power factor. The current is pulsed through the DC load string 104 because current sufficient to turn ON the DC load string 104 flows only during intervals when the AC line voltage Vac(t) is greater than Von(min). That is, as the incoming AC line voltage Vac(t) increases (or ramps up) from zero to Von(min), essentially no current or power is actually delivered to the loads until the instantaneous AC line voltage Vac(t) reaches the minimum ON voltage Von(min). Once turned ON, current flows through the DC load string 104 until the instantaneous AC line voltage Vac(t) again falls below the minimum ON voltage Von(min). Accordingly, throughout the remainder of the line cycle (outside the Ton and Toff interval), no sufficient voltage is available to turn ON the DC load string 104, while during the AC line cycle between Ton and Toff (which are near the peak AC voltage) sufficient current Idc(t) flows through the DC load string 104, resulting in a pulsed current waveform.
In general, pulsed current waveforms have a high crest factor with high peak currents, with relatively low average power delivered to the load compared to current waveforms with high power factor. It is preferable to deliver the maximum power with the minimum current, which is equivalent to having the power factor equal to one, sometimes referred to as unity power factor. The penalties for low power factor include added loss in the power delivery system, reduced reliability of the DC loads 106 owing to high peak current stress, and distortion of the AC line voltage waveform owing to the high peak currents. These issues may seem small when considering the effect of a single load of this type, but the effects are potentially enormous when considering the proliferation of many such loads collectively, simultaneously connected to the same AC power grid.
An added consequence of the pulsed current waveform is that the DC loads 106 are pulse driven, resulting in pulsed “operation” of the DC loads 106. The consequence of the pulsed current waveform for the circuits of FIGS. 1 and 2A (and FIG. 2A in particular) is that in the case where the DC loads 106 are Light Emitting Diodes (LEDs), the circuit generates large flicker in the light output of the LEDs. Light is emitted (or the DC load 106 operates) only when sufficient current Idc(t) flows, during brief intervals around the peaks of the AC line cycle (as illustrated in FIGS. 2C and 2D). In the case of full-wave rectified AC voltage, the resulting light output flicker occurs at twice the AC line frequency. This flicker reduces the quality of the light output, and may even have harmful side effects in individuals susceptible to light flicker.
In addition, the approach depicted in FIGS. 1 and 2A results in a circuit that is not compatible with conventional phase-controlled circuits, a non-limiting example of which may include phase-controlled light dimmer circuits. Conventional phase-controlled light dimmer circuits were originally designed for use with incandescent bulbs. The impedance of an incandescent bulb is largely resistive in nature. This resistive impedance characteristic is compatible with well-known phase-angle control methods used in conventional phase-controlled light dimmers. The AC driven LED circuit shown in FIGS. 1 and 2A is generally not compatible with conventional phase-controlled light dimmers, because the impedance of the DC loads 102 in FIGS. 1 and 2A is inherently nonlinear.
FIG. 3 depicts a conventional DC load string 104 driven by a DC-DC converter used to control the current through the DC load string 104. The bridge rectifier 108 rectifies the incoming AC line voltage Vac(t), and the resulting full-wave rectified AC voltage Vdc(t) is applied to the input of a switch-mode DC-DC converter 110. Owing to its complexity and the presence of energy storage elements (e.g., capacitors, inductors, etc.) within, the switch-mode DC-DC converter 110 is capable of controlling both the incoming AC current and the outgoing DC current through the DC load string 104. Properly designed, the system is capable of efficiently providing DC current to the DC load string 104, while maintaining high power factor at the AC input. This capability does not come without cost. The switch-mode DC-DC converter 110 is complex, typically consisting of a switching transistor, capacitor, inductor, rectifier diode, and associated control circuits at a minimum. The inductor and capacitor serve as energy storage components, which are needed for power processing. These elements are typically large and affect reliability. In addition, the switching action of these circuits is a significant generator of electromagnetic interference (EMI). Mitigating the EMI issues requires still more components. In the end, although the performance of this system can be very good, the complexity and number of components make the realization of this approach large, costly, and less reliable than simpler alternatives.
In addition to the aforementioned issues, the system with the DC-DC converter 110 shown in FIG. 3 is also generally not compatible with conventional light dimmer circuits. When powered from a conventional phase-controlled light dimmer, these circuits can exhibit oscillatory behavior. This oscillatory behavior can manifest itself as low frequency flicker (e.g., at frequencies below twice the AC line frequency), and this may induce harmful side effects in individuals susceptible to light flicker. Clearly, there is a need for a simple load regulator circuit that overcomes the issues discussed above.
Accordingly, in light of the current state-of-the-art and the drawbacks to current regulators mentioned above, a need exists for a source and multiple loads regulator that would enable a load to operate reliably within a wide range of source voltage inputs, with improved power factor, minimum power loss (high efficiency), and low cost of production.